1. Field of the Invention
Embodiments of the present invention generally relate to apparatus and methods for remotely installing underwater tools on sub sea structures.
2. Description of the Related Art
Numerous challenges are encountered in offshore operations, such as oil and gas exploration, production, and transportation. One such challenge is minimizing or eliminating the vibration of sub-surface equipment and structures caused by currents and tidal action. Typical marine structures susceptible to damage by currents and tidal action include subsea pipelines, drilling and production lines, import and export risers, tendons for tension leg platforms, and other elongated, sub-surface, components and structures used in offshore operations.
The flow of water around a rigid body, such as subsurface equipment and/or structures creates a vortex on the reverse or downstream side of the rigid body. The vortexes depart or shed from the downstream side of the rigid body at a frequency that is proportionate to the velocity of the fluid flowing past the rigid body. Over a given body shape, higher velocity flows will create a higher rate of vortex shedding than lower velocity flows over the same body. The vortices created by the flow are shed on alternate sides of the rigid body (e.g. the first vortex will shed 90° clockwise from the direction of flow, the second 90° counter-clockwise from the direction of flow). The process of vortex shedding on alternate sides of the body places alternating, 180° opposed, forces (i.e. a vibration) on the subsurface equipment and/or structures. The frequency of the vibration will vary with the velocity of the water flowing past the subsurface equipment and/or structures.
This current induced vibration in marine elements is often referred to as “vortex-induced vibration,” or “VIV.” When the frequency of shedding the vortices is near the natural frequency of the marine element, harmonic resonance can result in potentially destructive levels of vibration.
Sub-surface shrouds, fairings, and/or strakes are commonly used on equipment and structures to prevent or minimize vortex-induced vibration. Ordinarily, strakes or other VIV-reduction devices are installed on the surface prior to deployment of subsurface equipment and/or structures. For existing structures, VIV-reduction devices are frequently deployed using divers for shallow water installations or using remotely operated vehicles (“ROVs”) for deep water installations.
ROVs are usually the preferred way to install devices subsea, especially in deeper waters. In deeper waters, human divers are exposed to potentially dangerous working conditions, which are not a factor with a machine. A ROV is an underwater robot that is usually controlled from the surface by an operator. Typical ROVs are equipped with hydraulic manipulators, a vision system, and a remote control system to allow the operator to maneuver the ROV to a desired location under water to perform its intended task.
U.S. Pat. Nos. 6,994,492; 6,695,539; 6,928,709; and 7,316,525 each disclose hinged or clam shell underwater devices that are manipulated by a ROV for installing a clam shell, VIV-reduction device underwater. Such hinged or “clam shell” underwater device requires a manipulator or clamp having mating ends that rotate bi-directionally upon a pivot point within a single plane. The mating ends of the clamp engage opposite ends of the clam shell, VIV-reduction device in an open position and then rotate or pivot bi-directionally within a single plane to a closed position thereby closing the VIV-reduction device about a subsea structure to be protected from VIV. Such clam shell design in the deployment tool is inherently complex, requiring tight tolerances on the tooling during manufacturing and assembly. Operationally, adequate space must be provided for the tool's pivoting arms to swing during opening and closing. Also, the force exerted by the clam shell tool design when closing is greatly reduced as there is a tong moment arm disadvantage between the operating mechanism and where the clam shell tool closes the VIV device. The existing clam shell designs are also bulky and difficult for an ROV to handle, and in some cases the clam shell tool must be supported by a topside vessel when in operation.
There is a need, therefore, for improved systems and methods for deploying underwater devices about sub-sea structures.